Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing polarity electrode configurations. In unipolar pacing, the pacing stimulation pulses are applied between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In bipolar pacing, the pacing stimulation pulses are applied between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
The energies of the applied pacing pulses must be above the pacing energy stimulation or capture threshold of the respective heart chamber to cause the heart muscle of that chamber to depolarize or contract. If an applied pacing pulse has an energy below the capture threshold of the respective chamber, the pacing pulse will be ineffective in causing the heart muscle of the respective chamber to depolarize or contract. As a result, there will be failure in sustaining the pumping action of the heart. It is therefore necessary to utilize applied pacing pulse energies which are assured of being above the capture threshold.
However, it is also desirable to employ pacing energies which are not exorbitantly above the capture threshold. The reason for this is that pacemakers are implanted devices and rely solely on battery power. Using pacing energies that are too much above the capture threshold represent a waste of energy and result in early battery depletion and hence premature device replacement. Capture thresholds are assessed at the periodic follow-up visits with the physician and the output of the pacemaker is adjusted (programmed) to a safety margin that is appropriate based on the results of that evaluation. However, capture thresholds may change between scheduled follow-up visits with the physician. A refinement of the technique of periodic capture threshold measurement by the physician is the automatic performance of capture threshold assessment (autocapture) and the automatic adjustment of the output of the pulse generator. Capture thresholds may be defined in terms of pulse amplitude, either voltage or current, pulse duration or width, pulse energy, pulse charge or current density. With the introduction of AutoCapture, the implanted pacing system periodically and automatically assesses the capture threshold and then adjusts the delivered output. It also monitors capture on a beat-by-beat basis such that a rise in capture threshold will be immediately recognized allowing the system to compensate. Initially, the compensation is in the form of a significantly higher output back-up or safety pulse and then by incrementing the output of the primary pulse until stable capture is again demonstrated. A pacing energy may then be set by adding a small working margin to the capture threshold to assure reliable pacing without rapid depletion of the battery. Without autocapture, a much larger “safety” margin would have to be set and while this may save some energy for the system, it is not as efficient as AutoCapture with a small working margin and continued monitoring in minimizing battery current drain and maximizing device longevity.
As is well known in the art, the capture threshold of a heart chamber can, for various reasons, change over time. Hence, pacemakers that incorporate autocapture are generally able to periodically and automatically perform autocapture tests. In this way, the variations or changes in capture threshold can be accommodated.
When a pacing pulse is effective in causing depolarization or contraction of the heart muscle, it is referred to as “capture” of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization or contraction of the heart muscle, it is referred to as “lack of capture” or “loss of capture” of the heart.
In one known autocapture test, the pulse generator applies a succession of primary pacing pulses to the heart at a basic rate. To assess the threshold, the output of the primary pulse is progressively reduced. The output of each successive pair of primary pacing pulses is reduced by a known amount and capture is verified following each pulse. If a primary pulse results in loss of capture, a higher output backup or safety pulse is applied to sustain heart activity. If two consecutive primary pulses at the same output level are associated with loss of capture, the system starts to increment the output associated with the primary pulse. The output of successive primary pacing pulses is then incrementally increased until a primary pacing pulse regains capture. The output of the primary pulse which regains capture is the capture threshold to which the safety margin is added in determining the pacing energy. In these methods, capture may be verified by detecting the evoked response associated with the output pulse, the T-waves, mechanical heart contraction, changes in cardiac blood volume impedance, or another signature of a contracting chamber.
There are a number of different reasons why the capture threshold may vary and be unstable. An unstable threshold might indicate, for example, a recently implanted lead which is mechanically unstable. A lead may be mechanically unstable, for example, if it has an insulation or conductor coil failure. Both of these conditions would cause threshold fluctuations. Further, a progressive rise in capture threshold might occur with a developing disease state. Correlation of capture thresholds has not previously been utilized to detect or monitor any of these conditions due to the sporadic nature of the threshold measurements. There is one recent case report in the medical literature (Formieles-Perez H, et al, Documentation of Acute Threshold Rise in Ventricular Capture Thresholds Associated with Flecainide Acetate, PACE 2002; 25.
The present invention addresses this issue. It provides a means by which capture thresholds may be utilized to determine if the capture thresholds are unstable to reveal conditions which may otherwise go unnoticed until further advanced and causing a clinical problem. The capture thresholds may be utilized over short time periods to reveal acute conditions such as mechanical instability of an implanted lead or over longer time periods to reveal chronic conditions such as increasing thresholds associated with disease state or mechanical dysfunction of the chronic pacing lead.